1. Field of the Invention
The present invention relates to a galvanometer-type scanner that is used to move mirrors during laser marking or the like. In more detail, the present invention relates to a galvanometer-type scanner with improved acceleration without increases in size or cost.
2. Related Art
A galvanometer-type scanner is used to move mirrors during laser marking or the like. Such devices need to perform positioning operations for minute angles, such as one degree, within a positioning time in the order of several hundred microseconds.
The acceleration of a galvanometer-type scanner is improved by increasing the torque to inertia ratio. When the inertia is fixed, it is important how soon the torque rises. As can be understood from the graph in FIG. 4 showing the exciting current waveform of a direct current motor at startup, when the motor current is controlled by current feedback, normally the rise in torque can be brought forward and the responsiveness can be improved by increasing the ratio (KE/L) of the induced voltage constant KE to the armature inductance L. In this graph, E is the power supply voltage and R is the armature resistance.
This is to say, as shown by the graph in FIG. 4, at startup during a period that is considerably shorter than-the electrical time constant L/R, the current i can be approximated to (E/L)t (which is to say, i≈(E/L)t), and if the torque constant for the torque T is set at KT, the torque constant and induced voltage constant KE are equal in a DC motor, so that the torque T can be expressed as
T=KTi=(KT/L)Et=(KE/L)Et 
In this equation, Et is limited by the capacity of the power supply, so that KE/L is the index for the rise of torque.
Conventionally, the number of poles of a limited rotation-type DC motor that is used in a galvanometer-type scanner is two. In this case, the number of turns N in the armature winding of the DC motor is reduced in order to bring forward the rise of torque and improve the acceleration characteristics. This is because the induced voltage constant KE is proportionate to the number of turns N (KExe2x88x9dN), and the armature impedance L is proportionate to the square of the number of turns N (Lxe2x88x9dN2), so that KE/L is inversely proportionate to the number of turns.
However, if the number of turns is reduced, the torque constant KT that is equal to the induced voltage constant KE falls, so that the induced current in the winding increases. As a result, the capacity of the power converting element and power supply circuit increases, and since there is also an increase in power loss, this is disadvantageous from the viewpoints of size and cost.
The present invention was conceived in view of the problem described above and it is an object of the present invention to provide a galvanometer-type scanner with improved responsiveness without causing increases in size and cost.
In order to achieve the stated object, the present invention is a galvanometer-type scanner that is required to perform positioning operations for minute angles of around one degree within a positioning time in the order of several hundred microseconds, which is to say, in uses where the acceleration is 100,000 rad/s2 or higher. In cases where an oscillation angle of the motor specification is no greater than 30 degrees and the electrical time constant (L/R) of the DC motor is longer than the required positioning time, the number of poles in the DC motor is four.
With the galvanometer-type scanner of the present invention, the armature inductance L can be set at one half or less for the same induced voltage constant compared to the case of a bipolar DC motor where the external diameter of the rotor is the same. As a result, KE/L that is the index for the rising of the torque can be increased by a factor of two or more, so that the responsiveness of the DC motor can be improved.
It should be noted that by setting the number of poles of the DC motor at four, the effective oscillation angle is reduced, but since oscillation angles of 10 to 30 degrees are normally used when moving a mirror, this reduction in the effective oscillation angle is not problematic.